In powder metallurgy, very small diameter (&lt;1 .mu.m to 200 .mu.m) powder particles are charged into the die cavity of die presses and then compressed into parts or shapes. After initial compression these parts are typically less than 100% dense and the powder particles are much less than 100% bonded together. In many instances the relative number of "welded" points between particles is very low, producing a part that is considerably fragile. The strength of such a part is commonly called the "green" strength and is typically 50% or less than the strength of a 100% well bonded part. In order to obtain a fully bonded, in many instances denser part, this green formed part is then sintered at high temperature. In many cases a lubricant is added to the powder before pressing. In this circumstance, the lubricant has to be removed by a "delubing": burn off at a temperature below the sintering temperature. Typically, the temperatures used for sintering are at an appreciable fraction of the melting point (Tm) of the compressed metals or alloys, usually above 0.8 Tm. Sintering collapses the internal porosity of the green part and eventually results in a dense but usually less than a 100% well bonded part. The part is however, distorted in shape from the green part as pressed, mostly due to changes in density, and in some cases due to phase changes that may occur during sintering. Thus, present powder metallurgy methods are limited in that they cannot be used to produce parts having complex geometries and those that are made from materials that undergo undesirable phase changes at the high sintering temperatures necessary for bonding particle to particle.
For some active alloy powders which tend to oxidize readily in air, such as aluminum and titanium alloys, powder metallurgy (i.e., handling, consolidating and/or pressing) must be done with great care to avoid explosion. In Japan, aluminum powder metallurgy has been banned for this reason.
This explosion hazard is due to the extreme tendency of bare aluminum powder to oxidize and the usual air formed oxide prevents aluminum from cold-welding to itself. Water atomized aluminum has an even thicker surface oxide and consequently this oxide is incorporated into the structures of parts into which the aluminum powder may be used. Its presence degrades the thermal characteristics and other properties of the aluminum part. Hence, high temperature sintering is required to promote bonding of aluminum particles to each other.
When metal foils or sheets are used as starting materials, consolidation is often done by hot roll bonding. The hot rolling breaks up the naturally occurring oxide on the surface of the material, thereby enabling the surfaces of the powder particles (foils or sheets) to weld together at a sufficient number of contact points so as to provide adequate adhesion between the individual particles, sheets or foils. An example of such a hot roll bonding process can be found in U.S. Pat. No. 5,384,087 to Scorey. Such processes are not always satisfactory because they result in final structures having oxides incorporated therein. The significant amount of deformation required to break up these oxides can cause high internal stress which in turn require annealing and result in shape distortion.
In certain technologies parts or coatings are "printed" onto a substrate when, liquid metals, ceramics or mixtures are heated to extreme temperatures and projected onto a substrate with sufficient velocity to cause the material to weld to itself and to the substrate upon impact. Such processes are however limited, since they do not allow for precise control over the deposition process and hence cannot be used to produce printed parts which require great precision such as those used in micro-electronics and micro-imaging. These processes further allow oxides and porosity to be incorporated into the final deposited part. Additionally, since the temperature limits of apparatuses used to project the materials onto the substrates do not go high enough to permit the liquefaction of a number of materials, as for example copper, the type of materials that can be deposited using thermal spray technologies and hence the final product is limited.